1. Field of the Invention
The present invention relates to a method of transmitting an orthogonal frequency division multiplexing (OFDM) signal and a transmitter and receiver thereof.
2. Description of the Related Art
A macro diversity reception technique is known as one of the techniques for reception in a wireless communication system. In macro diversity, the same pilot signal and the same data signal are transmitted from a plurality of transmitters by the same carrier frequency. Such signals transmitted from each transmitter are received and combined at a receiver to obtain a predetermined gain. R1-051300, “MBMS transmission in E-UTRA”, LG Electronics, 3GPP TSG RAN WG1#43, Nov. 7-11, 2005 discloses a frame structure for performing macro diversity in an OFDM wireless communication system where signals that adapt and do not adapt macro diversity reception coexist. In the frame structure the pilot signals are scrambled using a scrambling pattern set for each transmitter. Data signals are not scrambled. The scrambling patterns are set so as to establish an orthogonal or pseudo orthogonal relation with respect to each other. According to this frame structure, since the pilot signal transmitted from a certain transmitter is in an orthogonal or pseudo orthogonal relation with signals transmitted from other transmitters, the receiver side is able to separate the signal transmitted from the certain transmitter from the signals from other transmitters. Accordingly, the pilot signal can be used for channel estimation of signals which do not adapt macro diversity reception.
To perform macro diversity reception for the signals transmitted in the frame structure, a two-step process is required at the receiver. In the first step, each channel response from each transmitter to receiver is estimated individually using the orthogonal or pseudo orthogonal relation. In the second step, the sum of each individually estimated channel response is used to compensate channel distortion of the received data signal by equalization, i.e. channel equalization. In this manner, channel equalization can be carried out for data signals arriving from each transmitter via each channel to the receiver.
For the receiver side to perform macro diversity reception for the signals of the frame structure, it is necessary to estimate each channel response from each transmitter to receiver individually. Originally, only the sum of each channel response is required for channel equalization in macro diversity reception. However, in the frame structure, it is necessary to obtain each channel response individually for channel equalization, which, consequently, increases the calculation amount.
Meanwhile, in the process of estimating each channel response individually, a desired channel response can be taken out alone by canceling out the other channels using the orthogonal or pseudo orthogonal relation. However, in some cases, channel distortion may weaken the orthogonality or the pseudo orthogonality, and the effectiveness to cancel out other channel responses may be reduced. In such case, the other channel responses are subject to complex addition as interference on the desired channel response, deteriorating accurate estimation of the desired channel response.
Further, in the process of estimating each channel response individually, the receiver is required to recognize all scrambling patterns included in the received signal. In the case where the receiver does not recognize a part of the scrambling pattern, the receiver will not be able to estimate the channel response from the transmitter to the receiver, and the reception performance deteriorates. Moreover, when the receiver recognizes a scrambling pattern not included in the received signal by mistake, the estimate value of a channel response generated by such scrambling pattern only includes interference. Thus, again, the reception performance deteriorates. For the receiver side to recognize the scrambling pattern, it is necessary to control information on which signal corresponding to which scrambling pattern is being received at the receiver side. Therefore, the control of the receiving side becomes complicated.